1. Field of the Invention
The present invention relates to a semiconductor quantum well laser having a low threshold current density and, more particularly, to a semiconductor quantum well laser lasing at a wavelength of around 1.3 .mu.m.
2. Description of the Related Art
A 1.3 .mu.m semiconductor laser lasing at a wavelength of around 1.3 .mu.m and composed of indium phosphide/indium gallium-arsenide phosphide (InP/InGaAsP) is generally used for a light source of a subscriber line in an optical communication network. This type of the semiconductor laser, however, has a disadvantage in a high threshold current density rated at up to 0.5 kA/cm.sup.2. Accordingly, a 1.3 .mu.m semiconductor laser is desired to lase at a lower threshold current, in view of power saving.
On the other hand, in the case of a 1.5 .mu.m InP/InGaAsP semiconductor laser lasing at a wavelength of around 1.5 .mu.m, it is known that a semiconductor laser of this type has excellent characteristics of a low threshold and a high quantum efficiency, where the quantum well active layers are subjected to tensile strain.
For example, it is reported in literatures that a quantum well laser lasing at a wavelength of around 1.5 .mu.m exhibits a minimum threshold current density as low as 90 ampere/cm.sup.2 (A/cm.sup.2) by applying InGaAs compound having a tensile strain of around 1.5% for quantum well layers. Examples of the literatures include "Low threshold 1.5 .mu.m tensile-strained single quantum well lasers". Electron, lett., 27, pp 1414-1416, 1991, presented by C. E. Zah, R. Bhat, B. Pathak, C. Caneau, F. J. Favire, N. C. Andreadakis, D. M. Hwang, M. A. Koza, C. Y. Chen, and T. P. Lee and "Submilliamp threshold current (0.62 mA at 0.degree. C.) and high output power (220 mW) 1.5 .mu.m tensile strained InGaAs single quantum well lasers", Electron Lett., 28, pp 829-830, 1992, presented by P. J. A. Thijs, J. J. M. Binsma, L. F. Tiemeijer and T. van Dongen.
This literature cast light on a fact that tensile strain applied on InGaAsP layers serving as quantum well layers in a semiconductor laser affects the band structure of the valence band of the quantum well lasers; as a result, the energy levels of light positive holes and heavy positive holes display a wider spread therebetween; consequently, there arise two advantages: a lower laser threshold and improved operational efficiency.
As for 1.3 .mu.m InGaAsP quantum well laser active layers, however, tensile strains above 1.0% are not introduced thereinto because it is believed that only a tensile strain exceeding 1.0% can produce phase-separation of InGaAsP compound into InP, GaAs etc. to thereby destroy the crystalline characteristics, as described in "III-V Group Semiconductor Phase Separation" presented by H. Nagai, S. Adachi and T. Fukui, published from Cornoa Corp, pp117-118, in which it is stated that InGaAsP compound suffers from phase-separation at a temperature within the curve of the isothermal line for the tensile strained InGaAsP, as will be detailed later.